1. Field of the Invention
Embodiments of the present invention generally relate to light-emitting diodes (LEDs) and methods for fabricating the same.
2. Description of the Related Art
Advances in light emitting diode (LED) technology have resulted in LEDs with characteristics of small volume, light weight, high efficiency, and long life. These LEDs have seen great advances in different monochromatic color output, such as red, blue and green. Single color LEDs can be used as a backlight in a special display, for instance, in mobile phones and light crystal displays (LCDs).
Recently, various attempts have been made to make white light sources by using light emitting diodes. Because the light emitting diode has an emission spectrum well-suited to generate monochromatic light, making a light source for white light may involve arranging three light emitting components of red (R), green (G), and blue (B) near each other while diffusing and mixing the light emitted by them. When generating white light with such an arrangement, there has been the problem that white light of the desired tone cannot be generated due to variations in the tone, luminance, and other factors of the light emitting component. Also, when the LEDs are made of different materials, electric power required for forward biasing differs from one light emitting diode to another, making it necessary to apply different voltages to different light emitting components, which leads to complex drive circuitry. Moreover, because the light emitting components are semiconductor light emitting components, color tone is subject to variation due to differences in temperature characteristics, chronological changes, and operating environment. Unevenness in color may also be caused by failure to uniformly mix the light emitted by the light emitting components. Thus, LEDs are effective as light emitting devices for generating individual colors, but a satisfactory light source capable of emitting white light by using LEDs has not been obtained so far.
U.S. Pat. No. 5,998,925 discloses a white light emitting diode having a light emitting component that uses a semiconductor as a light emitting layer and a phosphor, which absorbs part of the light emitted by the light emitting component and emits light of a wavelength different from that of the absorbed light. The light emitting layer of the light emitting component is a nitride compound semiconductor, and the phosphor contains garnet fluorescent material activated with cerium—which contains at least one element selected from the group consisting of Y, Lu, Sc, La, Gd, and Sm and at least one element selected from the group consisting of Al, Ga, and In—and is subject to less deterioration of its emission characteristics, even when used with high luminance for a long period of time.
FIG. 1 shows an LED as disclosed in the '925 patent. This LED is a lead-type LED having a mount lead 2 and an inner lead 4, wherein a light emitting component 8 is installed on a cup 6 of the mount lead 2, and the cup 6 is filled with a coating resin 14 that contains a specified phosphor to cover the light emitting component 8 and is molded in resin. An n-electrode and a p-electrode of the light emitting component 8 are connected to the mount lead 2 and the inner lead 4, respectively, by means of wires 12. In the LED constituted as described above, part of the light emitted by the light emitting component (LED chip) 8 (hereinafter referred to as LED light) excites the phosphor contained in the coating resin 14 to generate fluorescent light having a wavelength different from that of LED light, so that the fluorescent light emitted by the phosphor and the LED light that is output without contributing to the excitation of the phosphor are mixed and output. As a result, the LED also outputs light having a wavelength different from that of the LED light emitted by the light emitting component 8.
FIG. 2 shows a chip embodiment as disclosed in the '925 patent. The chip-type LED 26 is installed in a recess of a casing 22, which is filled with a coating material containing a specified phosphor to form a coating 28. The light emitting component 26 is fixed by using an epoxy resin or the like which contains Ag, for example, and an n-electrode and a p-electrode of the light emitting component 26 are connected to metal terminals 20 installed on the casing 22 by means of conductive wires 24. In the chip-type LED constituted as described above, similar to the lead-type LED of FIG. 1, fluorescent light emitted by the phosphor and LED light that is transmitted without being absorbed by the phosphor are mixed and output, so that the LED outputs light having a wavelength different from that of LED light emitted by the light emitting component 26. This type of conventional LED suffers from a color ring when used for emitting white light, whereby the color of the emitted light is bluer towards the middle and tends toward yellow at the edges near the casing 22.
U.S. Pat. No. 6,642,652 discloses a light source that includes a light emitting device—such as a III-nitride LED where Group 3 (III) includes such elements as Al, Ga, and In—covered with a luminescent material structure, such as a single layer or multiple layers of phosphor. Any variations in the thickness of the luminescent material structure are less than or equal to 10% of the average thickness of the luminescent material structure. In some embodiments, the thickness of the luminescent material structure is less than 10% of a cross-sectional dimension of the light emitting device. In some embodiments, the luminescent material structure is the only luminescent material through which light emitted from the light emitting device passes. In some embodiments, the luminescent material structure is between about 15 and about 100 microns thick. The luminescent material structure is selectively deposited on the light emitting device, for example, by stenciling or electrophoretic deposition.
An LED coated with phosphor according to the '652 patent is illustrated in FIG. 3. This LED includes an n-type region 44 formed on a substrate 42, such as sapphire, SiC, or a III-nitride material. An active region 46 is formed on the n-type region 44, and a p-type region 36 is formed on the active region 46. The n-type region 44, the active region 46, and the p-type region 36 are typically multilayer structures. Portions of the p-type region 36, the active region 46, and the n-type region 44 are etched away to expose a portion of n-type region 44. A p-type contact 34 is deposited on the p-type region 36, and an n-type contact 38 is deposited on the exposed portion of the n-type region 44. The LED is then flipped over and mounted to a sub-mount 30 by a material 32, such as solder. The luminescent material structure 40, such as phosphor, is deposited using electrophoresis to surround the LED at the individual die level.
U.S. Pat. No. 6,744,196 discloses thin film LED devices composed of LED chips that emit light at a first wavelength and a tinted thin film layer over the LED chip that changes the color of the emitted light. For example, a blue-light emitting LED chip can be used to produce white light. The tinted thin film layer beneficially consists of ZnSe, CeO2, Al2O3, or Y2O3Ce that is deposited using a chemical vapor deposition (CVD) process, such as metal organic chemical vapor deposition (MOCVD), atomic layer chemical vapor deposition (ALD), plasma enhanced MOCVD, plasma enhanced ALD, and/or photo enhanced CVD. As shown in FIG. 4, an n-contact 50 is positioned below a reflective layer 52. A tinted layer (a phosphor layer) 53 is positioned above the reflective layer 52. Next, a first passivation layer 54 is formed, and a p-type semi-transparent contact 56 is formed. A second passivation layer 58 is formed above the first passivation layer 54 and contact 56. A conductive wire 60 is connected to a p-pad 62, which is positioned above the p-lead 64.
Accordingly, what is needed is an improved semiconductor light source capable of emitting white light.